When photographer Lynn Johnson was working on this month’s National Geographic cover story on marijuana, she was surprised how addictive it became. Not the drug, mind you, but the controversial and convoluted medical world that legalization has spawned. So much so, that even after shooting on-and-off for six months for the magazine assignment, she couldn’t entirely let go of the story.

[Hear Johnson reveal what it was like to shoot the marijuana story in the video above.]

“I wasn’t prepared for the passion and depth of feeling that people associated with this plant,” said Johnson. She has continued photographing families with medically challenged children since finishing her assignment last December.

“I felt like what we do for the magazine is to